1. Field of the Invention
This invention relates to a method of decomposing halogenated aliphatic hydrocarbon compounds or aromatic compounds and also to an apparatus to be used for such a method. The invention also relates to a method of clarifying exhaust gas and an apparatus to be used for such a method.
2. Related Background Art
With developing industries, a large amount of aromatic compounds and halogenated aliphatic hydrocarbon compounds has been used, and the disposal of such compounds has become a serious problem. In particular, halogenated aliphatic hydrocarbon compounds causes great concern over environmental pollution, thus extensive research has been conducted for safe disposal of these compounds. For example, chlorofluorocarbons (CFCs), a kind of halogenated aliphatic hydrocarbon, have been used for sprays and as coolants in a huge amount, becoming a global problem as a cause of ozone holes in the ozone layer. Likewise, trichloroethylene (TCE), tetrachloroethylene (PCE) and other chlorinated aliphatic hydrocarbons have been abundantly used in various industries as cleansing solvents for metal parts, semiconductor parts and textiles and also as reaction solvents. However, their toxicity, such as mutagenicity and carcinogenicity have been pointed out, and it is required to ban their use completely, and to make the already used solvents harmless. Additionally, the compounds already released into the natural environment have been contaminating river water, ground water and soil. Therefore, development of new technologies for economical and efficient removal of the contaminants from the environment are in great need, and a variety of new technologies have been proposed for such applications.
For example, a combustion process has been proposed to decompose chlorinated aliphatic hydrocarbon compounds. Although this process is relatively simple, there is a new concern that decomposition products of chlorinated aliphatic hydrocarbons, i.e., hydrogen chloride and chlorine, would react with other organic compounds during combustion process to produce more toxic substances such as polybiphenyl chloride and dioxin. Additionally, the high temperature treatment is a drawback in view of energy consumption.
Also techniques of using an oxidizing agent or a catalyst for decomposing chlorinated aliphatic hydrocarbon compounds have been proposed. Specific examples include a process of decomposing harmful hydrocarbons by means of ozone (Japanese Patent Application Laid-Open No. 3-38297), a process of wet oxidation decomposition at high temperature under high pressure and a process of oxidization decomposition with hydrogen peroxide or a salt of iron (Japanese Patent Application Laid-Open No. 60-261590).
It is also proposed a process using sodium hypochlorite as an oxidizing agent (U.S. Pat. No. 5,611,642), or a combination use of sodium hypochlorite and UV irradiation (U.S. Pat. No. 5,582,741). It is also proposed a process where a suspension of a photocatalyst (fine particles of an oxide semiconductor such as titanium oxide) in liquid chlorinated aliphatic hydrocarbons is prepared under alkaline conditions and decomposing the latter by light irradiation (Japanese Patent Application Laid-Open No. 7-144137). A catalytic oxidation degradation process using an oxide such as platinum oxide, alumina and zirconium oxide is also proposed (H. Ichikawa et al., Japanese Patent Application Laid Open No. 6-31135).
Also, a photolysis process of decomposing chlorinated aliphatic hydrocarbon compounds by irradiating with UV light in a gaseous phase without an oxidizing agent is proposed (H. Seki et al, xe2x80x9cContaminated Groundwater and Soil: Present Condition and Countermeasurexe2x80x9d, ed. by Kansai Branch of Japan Water Environment Society and Environmental Technology Research Institute, 1995; Japanese Patent Application Laid-Open No. 8-243351).
It is known that chlorinated aliphatic hydrocarbons such as TCE and PCE are decomposed by microorganisms aerobically or anaerobically, and it has been tried to decompose such compounds and purify the natural environment by such a microbial process.
Japanese Patent Application Laid-Open No. 9-10554 and U.S. Pat. No. 5,340,555 describe a process of decomposing fluorinated aliphatic hydrocarbon compounds by means of a catalyst.
Japanese Patent Application Laid-Open No. 8-141367 discloses a process of decomposing CFCs by mixing them with fuel such as alcohol or ether and burning them in the presence of a catalyst.
U.S. Pat. No. 5,393,394 discloses a process of decomposing CFCs by exposing them to ultraviolet light either directly or after dissolving them into a solvent. Japanese Patent Application Laid-Open No. 3-074507 describes a method of reductively decomposing CFCs in mild conditions of room temperature and atmospheric pressure by contacting CFCs with an electrode in an electrolytic vessel.
As proposed techniques for decomposing the other organic compounds, e.g., hardly decomposable aromatic compounds such as those having a biphenyl bond and/or a biphenyl skeleton, there are (1) combustion, (2) decomposition by UV or radiation exposure and (3) decomposition using microorganisms. As a method for decomposing biphenyl compounds under mild conditions of room temperature and atmospheric pressure, there is a process wherein such compounds are decomposed by irradiating them with UV light directly or after dissolving them into a solvent. For example, Japanese Patent Publication No. 52-47459 discloses a method of rendering polychlorinated biphenyl harmless by dissolving it into alkaline alcohol, removing oxygen from the solution and then exposing the solution to ionizing radiation or UV light. Other known techniques for rendering PCB harmless include those described in Japanese Patent No. 919505 and Japanese Patent Applications Laid-Open Nos. 49-45027 and 57-166175.
Japanese Patent Application Laid-Open No. 7-000819 discloses a method for removing polychlorinated biphenyl etc. by utilizing the photocatalytic activity of titanium oxide. Japanese Patent Application Laid-Open No. 8-000759 describes a photodecomposition apparatus and method for degrading polychlorinated biphenyl, where water and alkali metals are removed as the photolysis-inhibiting factors by applying a DC voltage to an electrode provided in a reaction cell during the process of photodecomposition of PCB by UV light.
As listed above, a number of methods for decomposing halogenated aliphatic hydrocarbon compounds or aromatic compounds have been proposed to date. However, the inventors of the present invention have found that most of these methods require a complicated apparatus for decomposition, and or a further decomposition process of the decomposition products. Thus, there is still a strong demand for eco-friendly technology for decomposing halogenated aliphatic hydrocarbon compounds or aromatic compounds.
The present invention was made based on the inventors"" new finding.
An object of the present invention is to provide a method of efficiently decomposing halogenated aliphatic hydrocarbon compounds or aromatic compounds in an eco-friendly manner with less possibility of producing decomposition products that will cause another environmental contamination, as well as an apparatus to be used for such a method.
Another object of the present invention is to provide a method for efficiently purifying an exhaust containing halogenated aliphatic hydrocarbon compounds or aromatic compounds and an apparatus to be used for such a method.
In a study for achieving the above objects, the inventors of the present invention found that functional water, e.g., acidic water, which is obtained through electrolysis of water and of which sterilizing effect (Japanese Patent Application Laid-Open No. 1-180293) and cleansing effect for the surface of semiconductor wafers (Japanese Patent Application Laid-Open No. 7-51675) have been reported, can remarkably accelerate the decomposition of halogenated aliphatic hydrocarbon compounds or aromatic compounds under light irradiation. The present invention is based on this finding.
Thus, according to an aspect of the present invention, there is provided a method for decomposing halogenated aliphatic hydrocarbon compounds or aromatic compounds, which comprises a step of contacting a medium containing at least one of halogenated aliphatic hydrocarbon compounds and aromatic compounds with functional water under irradiation with light, where the functional water is produced through electrolysis of water containing one or more electrolytes.
The reason why the presence of functional water can accelerate the decomposition of various halogenated aliphatic hydrocarbon compounds or aromatic compounds under irradiation with light is not clear. However, the inventors presume that the functional water produced by electrolysis of water containing one or more electrolytes such as sodium chloride typically contains hypochlorous acid or hypochlorite, which by turn generate chlorine radicals, hydroxyl radicals or superoxides by light irradiation to promote the above decomposition reaction.
According to another aspect of the invention, there is provided a method of decomposing halogenated aliphatic hydrocarbon compounds or aromatic compounds, characterized by a step of contacting a medium containing at least one of halogenated aliphatic hydrocarbon compounds and aromatic compounds with functional water containing hypochlorous acid, under light irradiation.
According to still another aspect of the invention, there is provided a method for purifying an exhaust gas, characterized by a step of contacting exhaust gas containing at least one of halogenated aliphatic hydrocarbon compounds and aromatic compounds with functional water generated by electrolysis of water containing one or more electrolytes, under irradiation, to decompose the halogenated aliphatic hydrocarbon compounds or the aromatic compounds.
According to a further aspect of the invention, there is provided a method for purifying an exhaust gas, characterized by a step of contacting the exhaust gas containing at least one of halogenated aliphatic hydrocarbon compounds and aromatic compounds with functional water containing hypochlorous acid, under light irradiation.
According to a further aspect of the invention, there is provided an apparatus for decomposing halogenated aliphatic hydrocarbon compounds or aromatic compounds, which comprises a container equipped with a pair of electrodes and a power source for applying a voltage to the electrodes, means for supplying the container with water containing one or more electrolytes, an irradiation means and means for supplying a medium containing at least one of halogenated aliphatic hydrocarbon compounds and aromatic compounds.
According to another aspect of the invention, there is provided an apparatus for decomposing halogenated aliphatic hydrocarbon compounds or aromatic compounds, which comprises a decomposition treatment vessel, means for feeding functional water generated by electrolysis of water, means for feeding the container with a medium containing at least one of halogenated aliphatic hydrocarbon compounds and aromatic compounds, and means for irradiating the decomposition vessel with light.
According to another aspect of the invention, there is provided an apparatus for decomposing halogenated aliphatic hydrocarbon compounds or aromatic compounds, which comprises a container for functional water which contains hypochlorous acid, means for supplying the container with a medium containing at least one of halogenated aliphatic hydrocarbon compounds and aromatic compounds, and means for irradiating the container.
According to still another aspect of the invention, there is provided an apparatus for purifying exhaust gas containing at least one of halogenated aliphatic hydrocarbon compounds and aromatic compounds, which comprises a reaction container provided with inlet and discharge ports for an exhaust gas and functional water and filled with a filler between the inlet ports and the discharge ports, means for introducing the exhaust gas into the reaction container, means for introducing the functional water into the reaction container and means for irradiating the inside of the reaction container, where the functional water is obtainable by electrolyzing water containing one or more electrolytes.
According to still another aspect of the invention, there is provided an apparatus for purifying an exhaust gas containing at least one of halogenated aliphatic hydrocarbon compounds and aromatic compounds, which comprises a reaction container provided with inlet and discharge ports for the exhaust gas and functional water and packed with a filler between the inlet and discharge ports, means for introducing the exhaust gas into the reaction container, means for introducing functional water containing hypochlorous acid into the reaction container and means for irradiating the inside of the reaction container.
Japanese Patent Application Laid-Open No. 8-281271 discloses a process for decomposing one or more dyestuffs contained in dyeing water in an electrolytic vessel by means of hypochlorous acid and/or hypochlorite ion generated by electrolysis.
xe2x80x9cWater Treatment Technologiesxe2x80x9d, vol. 37, No. 5 (1996), p.33 describes a process of treating waste dyeing water, utilizing an electrochemical reaction. More specifically, it describes an indirect electrolytic method for decomposing dye molecules, where an oxidizing agent is generated by electrolysis, which then decomposes the dye by its oxidizing activity. The oxidizing agent is mostly hypochlorous acid which is formed by the reaction between chlorine gas generated at the anode and hydroxy ions generated at the cathode during electrolysis of the waste water to which sodium chloride was added. However, neither of the article nor the patent document cited above describes that halogenated aliphatic hydrocarbon compounds and aromatic compounds can be decomposed by any of the proposed methods. Furthermore, none of them even suggests irradiation can accelerate the decomposition of such compounds.